1. Field of the Invention
The present invention relates to an apparatus for performing an ophthalmic operation by photocoagulation using a laser beam, and, more particularly, to an apparatus for performing an ophthalmic operation in which an affected part in the fundus of a patient's eye is irradiated by a spot laser beam emitted from a laser optical system while an oculist observes the fundus of the patient's eye covered with one of two types of contact lenses using an observing system including a slit-lamp microscope.
2. Description of Related Art
A conventional apparatus for an ophthalmic operation using a laser beam is shown in FIG. 8 in which the ophthalmic operation is conducted by introducing the laser beam for ophthalmic treatment into an observing system including a slit-lamp microscope.
In FIG. 8, a laser diode capable of emitting a high power laser beam with wavelength of 800 nm, that is used for ophthalmic treatment, and a He-Ne laser source for aiming a spot laser beam with wavelength of 633 nm on the fundus of a patient's eye, are provided in the laser beam light source box 1. The laser beams emitted from both the laser diode and the He-Ne laser source are transmitted to a housing 3 of a laser optical system through an optical fiber 2.
The housing 3 is attached through a hole 5 of the slit-lamp body (indicated by a dashed line) and used for mounting a diopter adjustment bar. The laser beam from an edge plane 18 of the optical fiber 2 is transmitted to a dichroic mirror 11 through lenses 6, 9 and 10. The dichroic mirror 11 has the characteristics of (1) reflecting the light beam with wavelength of 800 nm and (2) reflecting half of visible light and transmitting the remaining half thereof. As a result, the optical axis of the laser optical system becomes coaxial with the optical axis of the observing system that includes a slit-lamp microscope 4. The laser beam is focused on the affected part in the fundus of the patent's eye after being reflected by the dichroic mirror 11 to perform photocoagulation on the affected part.
During the ophthalmic operation by photocoagulation using the above apparatus, an oculist observes the patient's eye 13 through the dichroic mirror 11 by the slit-lamp microscope 4. However, because of a very narrow distance between inherent objective plane 14 (focused plane) in the slit-lamp microscope 4 and the housing 3 of the laser optical system, it is necessary to obtain a working distance between the laser optical system and the patient's eye 13. This is accomplished by arranging a concave lens 15 on the light path of the observing system to shift the objective plane 14 to the position indicated by numeral 16.
More specifically, the position indicated by numeral 16 is located slightly at the right side, toward the slit-lamp microscope 4 due to the effect of refractive indices of both the patient's eye 13 and the medium of the contact lens 12. Further, the concave lens 15 is arranged to be inclined against the optical axis of illuminating light source 17 to prevent light reflected by the concave lens 15 from entering into the visual field of the observing system since illumination of the patient's eye 13 is conducted by the illuminating light source 17.
The edge plane 18 of the optical fiber 2 and the new objective plane 16 formed by the arrangement of the concave lens 15 are mutually conjugated through the lenses 6, 9 and 10. Therefore, the laser beam is focused on the fundus of the patient's eye 13 when the visual field of the observing system is focused on the fundus of the patient's eye 13 by adjusting the position of the slit-lamp microscope 4.
As described above, ophthalmic treatment by the laser beam is conducted while the contact lens 12 is placed over the patient's eye 13. Two types of contact lenses are used during the ophthalmic treatment. FIG. 8 shows an embodiment using a first type of contact lens 12.
In FIG. 9(a), the optical characteristics of the first type of contact lens 12 are shown. The laser beam reflected by the dichroic mirror 11 is focused on the fundus of the patient's eye 13 through the first type of contact lens 12. The first type of contact lens 12 is formed by cutting a cone body made of glass or acrylic resin so that the part of the lens 12 contacting the patient's eye 13 has a concave shape with a curvature the same as the curvature of the cornea of the patient's eye 13. By having the concave shape of the lens 12 contact the cornea of the patient's eye 13, an underserved effect caused by the refracting power of the cornea is prevented. Thus, the laser beam reflected by the dichroic mirror 11 is approximately aligned toward the fundus of the patient's eye 13 through the first type of contact lens 12.
In FIG. 9(b), the optical characteristics of a second type contact lens 12' is shown. This type of contact lens is more widely used than the first type of contact lens 12 in ophthalmic treatments. The second type of contact lens 12' focuses the fundus image of the patient's eye 13 on a plane 20 located in front of the patient's eye 13 and is advantageously capable of observing a fundus area that is wider than the first type of contact lens 12 based on the optical characteristics.
When the second type of contact lens 12' is used instead of the first type of contact lens 12 in the apparatus shown in FIG. 8, it is necessary to move the objective plane 16 to the plane 20 because the second type of contact lens 12' focuses the fundus image on the plane 20. Accordingly, the slit-lamp microscope 4 should be directed away from the patient's eye 13, in order to have the objective plane 16 coincide with the plane 20, because the patient's head 23 is substantially fixed to a headrest 22 and the headrest 22 is fixed to a table 21 on which the slit-lamp microscope 4 is mounted.
However, the movable range of the slit-lamp microscope 4 on the table 21 is very limited. Thus, an oculist is not able to keep the slit-lamp microscope 4 sufficiently from the patient's eye 13 in addition to the condition that the newly set objective plane 16 is moved from the objective plane 14, inherent in the slit-lamp microscope 4, by providing the concave lens 15 in the optical axis of the observing system, as described above. As a result, an oculist is not able to observe the fundus of the patient's eye 13.
To resolve this problem, it is possible to shorten the distance along which the new objective plane 16 is to be moved from the inherent objective plane 14, by weakening the refracting power of the concave lens 15. However, if the distance is shortened to an extent such that the second type of contact lens 12' becomes usable for the apparatus, the first type of contact lens 12 may not be usable because the space between the first contact lens 12 and the housing 3 of the laser optical system may become too small.
It is also possible to compensate for the limited movable range of the slit-lamp microscope 4 by modifying the connecting part between the headrest 22 and the table 21 to be movable in an opposite direction from the slit-lamp microscope 4. However, to realize this modification, it will be necessary to change the basic system of the slit-lamp microscope 4, and as a result, the slit-lamp microscope system will become structurally complex and cost-consuming.
Further, it is conceivable to provide a convex lens 24 at a position where the laser beam light path and the observing light path become coaxial for the second type of contact lens 12', as shown in FIG. 10. The convex lens 24 will compensate for the limited movable range of the slit-lamp microscope 4 by moving the new objective plane 16 toward the slit-lamp microscope 4. However, it would be very difficult to observe precisely the fundus of the patient's eye 13 because the reflected light from the convex lens 24 will enter the observed visual field, unless the convex lens 24 is arranged in an inclined or eccentric position against both the laser beam light path and the observing light path. In addition, a spot laser beam that is sufficiently small for use in the ophthalmic treatment of the fundus cannot be obtained because of a lens defect such as coma aberration, that is added to the laser beam light path when the convex lens 24 is arranged in the inclined or eccentric position.
Similar problems, as discussed above, exist when using the first type of contact lens 12 in an apparatus that is designed around the second type of contact lens 12'.